Application of the 118 nm single photon laser ionization technique to a molecular beam epitaxy machine is used for the first time to demonstrate direct desorption of As atoms from Si(100). Both As2 and As are the desorbing species from 1 ML of arsenic on silicon above 1000 K. This is in contrast to previously reported models that considered only dimer desorption. With a continuous flux of As4, the scattered and desorbing arsenic species from Si(100) are examined as a function of surface temperature (650–1200 K). Atomic desorption is large, 75%±19%, above 1000 K, and complete conversion of As4 to As2 and As occurs at 1200 K. The species selectivity of laser ionization time-of-flight mass spectroscopy has broader implications for GaAs growth.